Interferometers have become a central component in many remote sensing instruments. One such instrument is the Hyperspectral Interferometric Test Bed (HITB) sensor being developed by ITT Industries. The HITB sensor operates in the long wavelength infrared region (LWIR) and includes a compensated Michelson interferometer, an imaging telescope and a LWIR focal plane array (FPA). The Michelson interferometer uses retro-reflectors on an oscillating pendulum arm to provide a varying optical path difference between the two interferometer arms. The entire sensor is placed within a vacuum housing, where it is cooled to approximately 220° K. The HITB sensor views selected IR scenes through its vacuum window.
U.S. application Ser. No. 11/296,238 filed on Dec. 7, 2005, by the same inventor of the present application, describes an infrared interferometer that gives rise to etalons producing a rapidly oscillating pattern on the received scene energy. This oscillating pattern interferes with the desired scene energy and must be reduced or eliminated. In addition, very small changes in temperature of the interferometer causes significant changes in the internal optical path lengths that create the etalons. If changes in optical path length occur between calibrations of the interferometer, the transmission patterns of the etalons shift and result in radiometric errors.
Another problem described in U.S. application Ser. No. 11/296,238 is related to ghost images. Strong ghost images may result whenever two planar optical surfaces are in close proximity. Optical windows and beamsplitters commonly produce ghost images. For some applications there may be a maximum acceptable magnitude of ghost images. If the actual magnitude of a ghost image is found to exceed this maximum, the ghost magnitude must be reduced. Strategies for reducing ghost images include using high efficiency anti-reflection (AR) coatings where possible, wedging components to eliminate favorable etalon producing interference conditions, and choosing judiciously wedge and tilt angles so the ghost images are driven toward a wall away from the main image.
Wedging the interferometer plates greatly reduces the etalon transmission pattern. An IR ghost still exists, but the constructive interference from multiple reflections that gives rise to the etalon is reduced to small values. One conventional interferometer incorporates a 100 microradian wedge in its beamsplitter compensator. Another conventional interferometer incorporates a 1.2 milliradian wedge with an air wedge of 3.8 milliradians.
An example of a wedge is shown in FIG. 1. The figure depicts the output angles of ghost ray paths, when two surfaces are not parallel. As shown, wedge 10 includes two surfaces designated as 12 and 14. Path P1 and path P2 are primary image paths, and path T1 and path Rext are common ghost paths. The angular offsets of the ghost paths from the primary image paths are A3-A1 for the transmitted path and A2-A0 for the reflected path.
While wedges may reduce the etalon pattern, they cause other undesirable effects. The wedges may be large enough so that their chromatic aberration requires compensation in the optical system by including a window wedged in the opposite direction. This counter wedge may only partially reduce the aberration depending upon its location and tilt. Additionally, the components of the interferometer introduce anamorphic magnification.
U.S. application Ser. No. 11/296,238, which is incorporated herein by reference in its entirety, discloses ways of compensating a Michelson interferometer by using different types of compensators. One such compensator is shown herein in FIG. 2. As shown, Michelson interferometer 20 includes beamsplitter 22, compensator 24 and two flat retro-reflectors 25 and 26. The beamsplitter has two surfaces 22A and 22B, and the compensator has two surfaces (not labeled). As shown, the four surfaces are flat and parallel to each other. It will be understood, however, that these surfaces may be angled with respect to each other (thus forming wedges) and/or may be curved with respect to each other (thus forming surfaces having non-zero power).
In operation, incoming light (designated as input) enters beamsplitter 22, passing through input surface 22A. The passing light is split into first beam 27 and second beam 28, by beamsplitting surface 22B. The two beams 27 and 28 are shown in dashed and dotted lines, respectively. First beam 27 is reflected from beamsplitting surface 22B, whereas second beam 28 is transmitted through beamsplitting surface 22B.
Next, first beam 27 impinges upon and reflects from retro-reflector 25, thereby returning toward beamsplitter 22. The beam enters beamsplitter 22 at input surface 22A and exits beamsplitter 22 through beamsplitter surface 22B. The beam then enters and exits compensator 24 as an output beam.
Concurrently, second beam 28, after exiting beamsplitter 22 at beamsplitting surface 22B, enters and exits compensator 24. The second beam then impinges upon and reflects from retro-reflector 26, re-enters and re-exits compensator 24. Second beam 28 is next reflected from beamsplitting surface 22B of beamsplitter 22, again enters and exits the compensator, and finally arrives at the output, as shown. The two split beams 27 and 28 reunite and propagate approximately in the same direction, traveling toward an output detector (not shown) which detects the interference between the two beams.
A compensated Michelson interferometer, thus, has 4 opposing surfaces (2 surfaces of the beamsplitter and 2 surfaces of the compensator) resulting in 6 possible pairs of surfaces that may form etalons. These surfaces are typically wedged to reduce etalons. In addition, keeping the total radiometric error of a compensated interferometer below 1% requires keeping the temperature of the interferometer components constant to within a fraction of 1° K.
As will be explained, the present invention provides a Michelson interferometer that has advantages over conventional Michelson interferometers, because the interferometer of the present invention does not require compensation. As will be described, the present invention provides a Michelson interferometer having cube corners as retro-reflectors, where the cube corners provide self compensation for the interferometer.